1. Field of the Invention
A step-and-repeat type reducing projection exposure device (stepper) has been used to manufacture a semiconductor integrated circuit by a lithography process. In such an exposure device, an image of pattern formed on a photomask (hereinafter simply referred to as a mask) such as an original mask or a reticle is sequentially transferred to shot areas on a photosensitive substrate through a projection optical system.
2. Related Background Art
As a pattern to be transferred is minituarized, an aperture number of the projection optical system increases and a depth of focus of the projection optical system decreases. In order to keep the shot area of the photosensitive substrate within the depth of focus of a focal plane (focus point) of the projection optical system, the exposure device is equipped with an autofocus mechanism to focus the shot area of the photosensitive substrate to the focal point of the projection optical system. The autofocus mechanism may comprise a focus detection and transmission system for projecting an image of a slit pattern as a probe pattern to the shot area of the photosensitive substrate obliquely to an optical axis of the projection optical system, and a focus detection and reception system for receiving a reflected light of the image of the slit pattern and refocusing the image onto a photosensing device.
As the shot area of the photosensitive substrate moves along the optical axis of the projection optical system, the position of the image of the slit pattern on the photosensitive device changes. Thus, if the position of the refocused image when the shot area is positioned at the focal point of the projection optical system is known, a shift of the shot area from the focal point of the projection optical system can be detected. Based on the detection result, a table on which the photosensitive substrate is mounted is moved up and down to keep the shift within a predetermined range to conduct the autofocusing. For example, FIG. 7 shows a wafer 3 as the photosensitive substrate, and an exposure plane of the wafer 3 is divided into a plurality of shot areas 25. Each time the exposure is made to the shot area 25, the autofocusing is conducted.
In a pattern area of the mask in which a circuit pattern is formed, rectangular pattern units (for example, circuit devices) are periodically and finely arranged in rows and columns. Namely, the patterns for the mask are uniformly arranged in the pattern area, because more circuit devices can be formed in the shot area of the photosensitive substrate if the pattern area of the mask is fully utilized.
A plurality of layers of patterns are formed on the photosensitive substrate by repeating the lithography process such as exposure and development to the shot areas of the photosensitive substrate a predetermined number of times while a number of masks are exchanged. Fine uneveness is formed in each shot area of the photosensitive substrate by repeating the lithography process. In the semiconductor integrated circuit, since a level difference in the unevenness is much smaller than the depth of focus of the projection optical system, it does not cause a problem in detecting a shift of the shot area from the focal point of the projection optical system (focus detection).
However, the stepper is recently used not only for the manufacture of the semiconductor integrated circuit but also for the manufacture of a thin film head for a magnetic disk drive. In the manufacture of such a thin film head, a number of thin film heads, for example, 100 heads are formed in one shot area. For example, as shown in FIG. 8, in the thin film head, a difference (level difference) between a substrate plane and a top plane of the thin film head is relatively large, for example, approximately 30 .mu.m. In FIG. 8, numeral 26 denotes a projection which is periodically formed on a wafer as a thin film head, and patterns of the thin film head are formed in multi-layers on the projection 26. In order to form the uneven patterns having the large level difference, the unevenness is gradually formed on the photosensitive substrate by the lithography method while a number of masks are exchanged.
When the uneven pattern having a small pitch and a large level difference is to be formed, it is necessary to focus the shot area to be exposed to the focal plane of the projection optical system. To this end, as shown in FIG. 8, it is necessary to project a probe pattern 27 such as a slit pattern to a projection or a recess of the shot area to exactly detect the position of the projection or the recess relative to the focal plane of the projection optical system. However, if the area of the projection or the recess is smaller than the size of the probe pattern, the probe pattern is projected to the projection and the recess simultaneously so that the level of the projection or the recess cannot be exactly detected.